Green with Envy

What color will you see in Libra’s beta star?

By J. Alex Knoll

Thursday’s new moon leaves weekend skies bereft of excess light, highlighting the backdrop of stellar lights.
As the sun sets, the first lights you see are likely to see are above the southwest horizon, where Saturn and Spica hover just five degrees from one another. Both shine at first magnitude, but Saturn’s steady golden glow is an easy contrast to the blue-white twinkle of Spica, the lead star in the constellation Virgo. Look for Mars 10 degrees to the northwest, not quite as bright as of Venus or Spica but a distinct orange. Over the coming weeks Mars pulls closer to the two before crossing between them in mid-August.
Following Virgo is the constellation Libra, a good 20 degrees to the southeast. Look for it midway between Spica and the red-giant Antares of Scorpius farther to the east. While none of its stars are brighter than second magnitude, the shape of the scales is easy to discern. It’s two main stars are Zubeneschamali, the fulcrum of the scales, and Zubenelgenubi, which lies directly between Antares and Spica. The two stars appear about 10 degrees apart, the width of a fist extended at arms-length.
Zubeneschamali is the brighter of the two stars, barely. Yet Zubenelgenubi is technically the constellation’s alpha star, perhaps because it is perched directly along the ecliptic, the annual path of the sun, moon and planets through the sky. At first blush, the two appear much the same, but Zubeneschamali is 160 light years from us compared to the 77 light years separating us from Zubenelgenubi. Zubeneschamali is actually 5 times as luminous as Zubenelgenubi and 130 times more so than our sun. Zubeneschamali is a matter of controversy in the astronomy world, as some claim that it shines a pale emerald green, the only visible star to do so. However, as many people see Zubeneschamali as blue or even white. What will you see?